The invention relates to an apparatus and method of forming fibrous molding compositions for fiber-reinforced resin composites and, more particularly, to an apparatus and method for impregnating or admixing discontinuous fibers with dry powdered resin.
Fiber-reinforced resin composites are assemblies of fibers geometrically arranged within a resin matrix to obtain specific characteristics and properties. The overall properties of the composite are a function of many variables including the amount and type of fiber and matrix, the orientation and distribution of the fiber, the compositing technique, and the processing method utilized in fabricating the finished composite article.
In processes utilized to form composite articles having considerable and varying thickness and contours, it is desirable to utilize resin moldings containing discontinuous fibers to obviate a need for subsequent machining.
In the past, fibrous tow or roving has been passed through a liquid resin composition, dryed and chopped to provide a mass of loose resin-coated fiber segments. Wet resin impregnation of the fibers, however, requires the availability of a suitable solvent for the matrix material. Moreover, it has been found that some of the fiber segments impregnated by the wet resin invariably cohere requiring additional separation to optimize strength in the finished article. Wet impregnation also often requires the maintenance of resin bath levels or heating to maintain proper viscosity of the bath or both, and subsequent drying.
Dry impregnation or the mixing of discontinuous fibers with dry resin powders is likewise subject to difficulties. Powdered resins and fibers may be mixed in a tumbler or blender. However, in such apparatus the fibers tend to remain in discrete bundles resisting powdered resin penetration, and resulting in a non-homogenous mixture. Molding compositions prepared by such mixing techniques are necessarily produced by batching operations rather than by continuous production techniques.
Application of dry resin to continous fiber tows generally requires in-situ heating to assure coherence between the fiber and resin. Heating, however, may not be desirable or practical. Such techniques may not result in a uniform fiber-resin distribution.
Furthermore, preliminary process conditioning such as solvenating and heating may either limit the materials which can be utilized, or adversely affect the desired properties of the ultimate composite application.